Confession alert: I love geothermal! Quiet? Energy efficiency numbers through the roof (ground, actually)? It is such a smart idea!
That said, I am not a geothermal technical expert, so I consulted with some people who are and got some interesting responses. The idea is that you need adequate separation between the bores in order to provide sufficient soil to adsorb and supply heat. Trying to fit them in close quarters is analogous to cramming solar planels on a roof that is shaded a good portion of the day because of nearby trees ? you aren?t going to get the performance for which you paid.
A David Meyers, an International Ground Source Heat Pump Association Accredited Installer with Verde Sol-air Services in Arizona, said that he would not go less than 15 feet on center, no matter what the soil conditions.
Jerry Moore, an IGSHPA Certified GeoExchange Designer with Design Mechanical Inc. in Illinois, says he does not go less than 20 feet on commercial system design. However, residential load demands are lower than commercial and he knows residential installers who go as little as 10 feet on center.
The key is to utilize an Accredited Installer or a Certified GeoExchange Designer who has at least 10 years of experience and has done multiple houses in your area, as soil types vary considerably. Jerry also recommends doing a test bore hole to verify soil type. With correct inputs, they can use software to calculate the minimum system needs.
You can find accredited professionals through the International Ground Source Heat Pump Association.
You can get additional information through the Geothermal Heat Pump Consortium, which has an online forum.